English

• 1. INTRODUCTION

  Recognition and detection of small organic molecules have drawn more and more attention because of their important impact on environment^[1-3]. Volatile organic solvents, in particular, are highly toxic and can thus affect environment and human body more widely. Hence, there is a strong need to develop a quick response and simple identification method for the volatile organic solvents. Fluorescence recognition has received much attention because of its short response time, excellent sensitivity and simplicity^[4]. This technique has been applied to detect small organic molecules such as explosives, amines and acetonitrile^[5-11].

  Lanthanide metal-organic frameworks (Ln-MOFs), a new class of crystalline luminescence materials, have been of great importance due to their high luminescence efficiency, large Stokes shifts, narrow band emissions and long luminescence lifetimes^[12]. Ln-MOFs with high luminescence intensity can be rationally designed and synthesized via the appropriate choice of antenna ligands and Ln³⁺ ions^[13-16]. Owing to the electron (charge) transfer processes, the interaction of analytes to be tested with Ln-MOFs may alter the luminescence intensity, hence resulting in their emission color changes. Taking advantages of the obvious emission color changes, Ln-MOFs are able to probe small organic molecules, including organic solvents, dyes, explosives and antibiotics^[17-19]. However, for the detection of organic solvents, most are performed in liquid phase. Detection of organic solvent vapor by Ln-MOFs is less common. Herein, considering that both 3, 3′, 5, 5′-azobenzenetetracarboxylic acid (H₄abtc) bearing a photochromic azo group and 1, 10-phenanthroline (phen) are strong light-harvesting organic chromophores and are beneficial to improve the luminescence properties of Ln-MOFs, we used them as a bridging and a chelating ligands, respectively, to construct an Eu-MOF, formulated as (Me₂NH₂)[Eu(abtc)(phen)]·2H₂O (1). Compound 1 was fully characterized by elemental analysis, IR, TGA, and PXRD. It has good thermal stability and is also stable in aqueous solution. Moreover, 1 was found to be a selective and sensitive fluorescent probe for diethyl ether in solution or vapor state. It is worth mentioning that this is the first report for detecting diethyl ether vapor using Ln-MOFs through a luminescence enhancement mechanism.

  2. EXPERIMENTAL

  2.1 Materials and physical measurements

  All reagents were commercially purchased and used without further purification. Elemental analyses (C, H, and N) were carried out using an elemental Vario ELⅢ analyzer. Experimental powder X-ray diffraction (PXRD) was recorded on a Miniflex 600 diffractometer with CuKɑ radiation (λ = 1.5406 Å) at a scan speed of 1.6 º/min. Thermogravimetric analysis (TGA) was measured on an TGA/NETZSCH STA 449C analyzer at a heating rate of 10 ℃/min under nitrogen atmosphere. Infrared spectrum was recorded on a PerkinElmer Spectrum-One spectrophotometer using the KBr pellet. The emission spectra were obtained on an Edinburgh FLS fluorescence spectrophotometer.

  2.2 Synthesis of (Me₂NH₂)[Eu(abtc)(phen)]·2H₂O

  A mixture of Eu(NO₃)₃·6H₂O (0.1 mmol), 3, 3′, 5, 5′-azobenzenetetracarboxylic acid (H₄abtc, 0.05 mmol), phen (0.1 mmol), H₂O (1 mL), and DMA (5 mL) was placed in a 25 mL Teflon cup and heated at 150 ℃ for 2 days. Then, the reaction was slowly cooled down to room temperature to obtain orange block crystals. After filtration, the product was washed with ethanol and air-dried. Yield: 165 mg, 73% (based on Eu). Elemental analysis (%): calcd. for 1: C₃₀H₂₆N₅O₁₀Eu (768.50): C, 46.84; H, 3.38; N, 9.11. Found (%): C, 46.65; H, 3.13, N, 9.46. IR (cm^‒1): 1610 (vs), 1552 (s), 1437 (s), 1373 (vs), 1239 (w), 1145 (w), 1092 (m), 1015 (w), 932 (w), 855 (m), 798 (m), 721 (s), 625 (w), 490 (w), 465 (w), 420 (w).

  2 3 X-ray crystallographic study

  Single-crystal X-ray diffraction data for 1 were collected on a Rigaku Mercury CCD diffractometer equipped with graphite-monochromated MoKα radiation (λ = 0.71073 Å) at 293(2) K. Absorption correction was carried out by the multi-scan program. The structure was solved by direct methods with SHELXS-97^[20] and refined by the full-matrix least-squares method on F² using the SHELXL-2016 program^[21]. The contribution from disordered guest solvents was removed by using the SQUEEZE program implemented in PLATON^[22]. Selected bond lengths and bond angles are listed in Table 1. Crystallographic data have been deposited at the Cambridge Crystallographic Data Center.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (º) for Compound 1

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Eu(1)–O(1)	2.469(3)		Eu(1)–O(5)	2.483(3)		Eu(1)–O(8)b	2.394(3)
  Eu(1)–O(2)	2.558(3)		Eu(1)–O(6)	2.572(3)		Eu(1)–N(3)	2.625(4)
  Eu(1)–O(4)a	2.417(3)		Eu(1)–O(7)	2.354(2)		Eu(1)–N(4)	2.593(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Eu(1)–O(2)	52.04(8)		O(7)–Eu(1)–O(6)	71.71(8)		O(5)–Eu(1)–O(6)	51.87(8)
  O(1)–Eu(1)–O(5)	78.74(9)		O(7)–Eu(1)–N(3)	83.18(11)		O(5)–Eu(1)–N(4)	82.49(10)
  O(1)–Eu(1)–N(3)	146.08(10)		O(8)b–Eu(1)–O(1)	76.27(9)		O(6)–Eu(1)–N(4)	130.55(10)
  O(2)–Eu(1)–O(6)	104.98(9)		O(8)b–Eu(1)–O(4)a	137.39(10)		O(7)–Eu(1)–O(2)	135.94(9)
  O(2)–Eu(1)–N(4)	70.72(10)		O(8)b–Eu(1)–O(6)	136.92(8)		O(7)–Eu(1)–O(5)	123.44(9)
  O(4)a–Eu(1)–O(2)	134.55(9)		O(8)b–Eu(1)–N(4)	90.79(10)		O(7)–Eu(1)–O(8)b	79.66(9)
  O(4)a–Eu(1)–O(6)	73.77(9)		O(1)–Eu(1)–O(6)	71.15(9)		O(7)–Eu(1)–N(4)	145.15(11)
  O(5)–Eu(1)–O(2)	70.90(9)		O(1)–Eu(1)–N(4)	122.76(10)		O(8)b–Eu(1)–O(2)	74.44(9)
  O(5)–Eu(1)–N(3)	132.83(10)		O(2)–Eu(1)–N(3)	118.98(11)		O(8)b–Eu(1)–O(5)	144.99(9)
  O(6)–Eu(1)–N(3)	134.73(10)		O(4)a–Eu(1)–O(1)	144.23(10)		O(8)b–Eu(1)–N(3)	69.98(10)
  O(7)–Eu(1)–O(1)	87.65(9)		O(4)a–Eu(1)–O(5)	74.32(10)		N(3)–Eu(1)–N(4)	62.13(12)
  O(7)–Eu(1)–O(4)a	87.94(9)		O(4)a–Eu(1)–N(3)	68.14(11)
  Symmetry transformation: a: x + 1/2, y + 1/2, z; b: –x + 1, –y, –z

  3. RESULTS AND DISCUSSION

  3.1 Structure description

  Single-crystal X-ray diffraction analysis reveals that 1 crystallizes in the monoclinic system, C2/c space group. The SQUEEZE function in PLATON was used during the refinement due to the highly disordered solvents. TGA and elemental analyses indicate the presence of two free water molecules. The asymmetric unit of 1 consists of one crystallographically independent Eu³⁺ ion, one abtc^4‒ ligand, one phen ligand, one protonated dimethylamine cation and two free water molecules. As shown in Fig. 1, the nine-coordinated environment of Eu³⁺ is fulfilled by seven carboxylate oxygen atoms derived from the abtc^4‒ ligands and two nitrogen atoms from a phen ligand. The Eu–O bond lengths range from 2.356(4) to 2.573(4) Ǻ and the Eu–N bond lengths are 2.595(5) and 2.620(5) Å. The O–Eu–O bond angles range from 51.89(11) to 144.20(13)° and the N–Eu–N bond angle is 62.12(17)°, all of which are comparable to those reported in the related Eu³⁺ compounds^[23]. The four carboxylate groups of the H₄abtc ligand are all deprotonated and adopt three types of bridging modes: (κ¹-κ¹)-μ₁, κ¹-μ₁ and (κ¹-κ¹)-μ₂. Each abtc^4‒ ligand acts as a μ₅-bridge to link five Eu³⁺ ions (Fig. 1). In the structure of 1, two symmetry-related EuO₇N₂ polyhedra are assembled into a dimer by bridging carboxylate groups with the Eu···Eu distance of 5.8097 Å, and these dimers are linked by bridging abtc^4- ligands to form a 2D layer with rhombic windows (Fig. 2). Then a 3D framework of 1 is formed from these 2D layers via the linkage of abtc^4‒ ligands (Fig. 3). Unlike most lanthanide-abtc frameworks where the Ln³⁺ centers are usually seven- or eight-coordinate and their coordination spheres are fulfilled by one or two terminally coordinated solvent molecules, such as H₂O, DMF, DMA and DMSO, the Eu³⁺ center in 1, as also observed in its most related Tb-abtc compound, adopts a distorted tricapped trigonal prism geometry and is nine-coordinate without any coordinated solvent molecules^[24-26]. Furthermore, the coordination sphere of Eu³⁺ ion is completed by a chelating phen ligand, which plays an important role in the enhancement of stability.

  Figure 1

  

  Figure 1.  Coordination environment of Eu³⁺ ion and coordination mode of the ligand in 1

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  Figure 2

  

  Figure 2.  View of 2D layer of compound 1

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  Figure 3

  

  Figure 3.  View of 3D framework of compound 1. The phen ligands are omitted for clarity

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  3.2 Chemical and thermal stability of 1

  To confirm the phase purity, the powder X-ray diffraction of 1 was measured at room temperature. The diffraction pattern is consistent with the simulated one based on the single-crystal X-ray diffraction, implying phase purity of the as-synthesized sample (Fig. 4). Furthermore, the acid stability of 1 was tested by soaking the sample in aqueous solutions of various pH values (3 to 11) for one day. The XRD results showed that after soaking, the patterns of XRD remained the same, which indicated that there was no structural change. TGA result shows that the first weight loss from 30 to 160 ℃ with the loss of 4.91% is due to the departure of lattice water (calcd. 4.68%) (Fig. 5). The second weight loss from 160 to 340 ℃ with the loss of 5.47% is due to the removal of Me₂NH₂ cation (calcd. 5.99%). Further increase in temperature leads to the framework decomposition.

  Figure 4

  

  Figure 4.  PXRD patterns of 1 after immersion in aqueous solutions with pH ranging from 3 to 11 and after sensing

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  Figure 5

  

  Figure 5.  TGA curve of compound 1

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  3.3 Fluorescence sensing

  The solid-state fluorescence of 1 was recorded. Upon excitation at 290 nm, the emission spectrum consists of four peaks at 594, 614, 652 and 700 nm due to the ⁵D₀ → ⁷F_J (J = 1, 2, 3, 4) transition of Eu³⁺. The peak at 614 nm results from the hypersensitive transition ⁵D₀ → ⁷F₂ of Eu³⁺ and has the maximum intensity.

  In recent years, recognition and sensing of small molecules, in particular volatile organic molecules have provoked more and more attention. To probe its potential application as chemical sensor, the fluorescence emission of 1 in a range of common solvents (diethyl ether, water, formaldehyde, ethanol, methylbenzene, acetonitrile, methanol and acetone) was measured. As depicted in Fig. 6a, the emission intensity of 1 varies depending on the types of solvents. Obviously, the fluorescent intensity in diethyl ether increases about 3.4 times than that in water and formaldehyde, while in the rest solvents it decreases more or less. In addition, compound 1 has a good cyclic performance. After five cycles of repetition, the luminescence intensity does not change significantly (Fig. 6b). Therefore, compound 1 might be used for the selective detection of diethyl ether.

  Figure 6

  

  Figure 6.  (a) Luminescence intensity of 1 dispersed in different solvents, (b) Five cycle tests of 1 for sensing diethyl ether

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  The fluorescence enhancement of diethyl ether, as well as its good chemical stability, promotes us to study its detection ability for diethyl ether in vapor state. To avoid deterioration of diethyl ether, the sensing experiments were performed under indoor environmental conditions at room temperature. An open sample tube with finely ground sample of 1 was placed in a sealed container, containing diethyl ether (Scheme 1). The time-dependent fluorescent intensity of 1 shows a continuous enhancement upon exposure to diethyl ether vapor for 10, 30, 45 and 60 min (Fig. 7). The fluorescence enhancement efficiency reaches to a maximum in 60 min. These experimental results indicate that 1 could be a promising chemical sensor for detecting diethyl ether vapors.

  Scheme 1

  

  Scheme 1.  Diagram of sensing measurements for diethyl ether vapors

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  Figure 7

  

  Figure 7.  Fluorescence efficiency of 1 exposed in diethyl ether vapor for different time

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  3.3 Possible mechanism for fluorescence sensing

  The mechanism for detection of diethyl ether was further investigated. The PXRD pattern of 1 after sensing test is exactly the same as that of the simulated one, which suggests that the interaction of diethyl ether with 1 does not cause structural collapse. The enhancement of fluorescent intensity may be due to the interaction between diethyl ether and the framework. The presence of diethyl ether may replace some H₂O surrounding the mental center, which can decrease H-bond oscillation, hence resulting in fluorescence enhancement^{[27, 28]}.

  4. CONCLUSION

  In summary, we have successfully synthesized 1 via self-assembly of H₄abtc and phen ligands with Eu³⁺ under solvothermal condition. Compound 1 is a porous 3D framework and its application in detection organic solvent molecules was also explored. It displays an obvious fluorescence enhancement effect in diethyl ether. Further study demonstrated that 1 was also capable of detecting diethyl ether vapor.

  
  
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• 

  Figure 1  Coordination environment of Eu³⁺ ion and coordination mode of the ligand in 1

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  Figure 2  View of 2D layer of compound 1

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  Figure 3  View of 3D framework of compound 1. The phen ligands are omitted for clarity

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  Figure 4  PXRD patterns of 1 after immersion in aqueous solutions with pH ranging from 3 to 11 and after sensing

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  Figure 5  TGA curve of compound 1

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  Figure 6  (a) Luminescence intensity of 1 dispersed in different solvents, (b) Five cycle tests of 1 for sensing diethyl ether

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  Scheme 1  Diagram of sensing measurements for diethyl ether vapors

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  Figure 7  Fluorescence efficiency of 1 exposed in diethyl ether vapor for different time

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (º) for Compound 1

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Eu(1)–O(1)	2.469(3)		Eu(1)–O(5)	2.483(3)		Eu(1)–O(8)b	2.394(3)
  Eu(1)–O(2)	2.558(3)		Eu(1)–O(6)	2.572(3)		Eu(1)–N(3)	2.625(4)
  Eu(1)–O(4)a	2.417(3)		Eu(1)–O(7)	2.354(2)		Eu(1)–N(4)	2.593(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Eu(1)–O(2)	52.04(8)		O(7)–Eu(1)–O(6)	71.71(8)		O(5)–Eu(1)–O(6)	51.87(8)
  O(1)–Eu(1)–O(5)	78.74(9)		O(7)–Eu(1)–N(3)	83.18(11)		O(5)–Eu(1)–N(4)	82.49(10)
  O(1)–Eu(1)–N(3)	146.08(10)		O(8)b–Eu(1)–O(1)	76.27(9)		O(6)–Eu(1)–N(4)	130.55(10)
  O(2)–Eu(1)–O(6)	104.98(9)		O(8)b–Eu(1)–O(4)a	137.39(10)		O(7)–Eu(1)–O(2)	135.94(9)
  O(2)–Eu(1)–N(4)	70.72(10)		O(8)b–Eu(1)–O(6)	136.92(8)		O(7)–Eu(1)–O(5)	123.44(9)
  O(4)a–Eu(1)–O(2)	134.55(9)		O(8)b–Eu(1)–N(4)	90.79(10)		O(7)–Eu(1)–O(8)b	79.66(9)
  O(4)a–Eu(1)–O(6)	73.77(9)		O(1)–Eu(1)–O(6)	71.15(9)		O(7)–Eu(1)–N(4)	145.15(11)
  O(5)–Eu(1)–O(2)	70.90(9)		O(1)–Eu(1)–N(4)	122.76(10)		O(8)b–Eu(1)–O(2)	74.44(9)
  O(5)–Eu(1)–N(3)	132.83(10)		O(2)–Eu(1)–N(3)	118.98(11)		O(8)b–Eu(1)–O(5)	144.99(9)
  O(6)–Eu(1)–N(3)	134.73(10)		O(4)a–Eu(1)–O(1)	144.23(10)		O(8)b–Eu(1)–N(3)	69.98(10)
  O(7)–Eu(1)–O(1)	87.65(9)		O(4)a–Eu(1)–O(5)	74.32(10)		N(3)–Eu(1)–N(4)	62.13(12)
  O(7)–Eu(1)–O(4)a	87.94(9)		O(4)a–Eu(1)–N(3)	68.14(11)
  Symmetry transformation: a: x + 1/2, y + 1/2, z; b: –x + 1, –y, –z

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